1. Field of the Invention
The present invention relates to an optical coupling module suitable for use in terminal stations in optical telecommunications and also to a method of manufacturing the same.
2. Related Art
Recently, a project called "Fiber to the Home" has been in progress, which is extending optical fibers up to individual homes for optical communications with a large traffic capacity.
Under this project, multiplexed light having component beams of wavelength bands of 1.3 .mu.m and 1.55 .mu.m, for example, is sent to communication terminals of individual homes.
As optical elements in optical devices at terminal stations to receive multiplexed light such as mentioned above, it has been proposed to use CGH (Computer Generated Hologram) elements as described in the specification of Japanese Patent Application No. 9-147115 (referring to U.S. Ser. No. 09/081,080), for example.
The CGH elements are optical elements manufactured by using a computer program, and therefore those high-precision optical elements can be mounted in compact form on an optical substrate.
If the CGH elements are adopted, it becomes possible to mount optical elements in a compact form on a multi-layer structure of optical substrates. Those optical elements mounted include an optical demultiplexing element to separate the multiplexed light into rays of different wavelengths, or an optical coupling element to separate or couple a ray of one wavelength obtained by demultiplexing.
Therefore, by connecting optical functional elements, that is, a light-emitting element such as a semiconductor laser, and a light-detecting element such as a photodiode, to an optical device formed by a multi-layer structure, it is possible to produce an optical coupling module suitable for optical communication terminal stations capable of bi-directional communications.
For an optical coupling module mentioned above, a semiconductor substrate is generally used. One surface of a semiconductor substrate is polished to a mirror finish by a chemical and mechanical etching to improve the flatness. An optical functional element, such as a semiconductor laser or a photodiode is mounted on the mirror-finished surface of the semiconductor substrate. The optical device is supported on the semiconductor substrate that has the optical functional elements mounted thereon so that the optical functional elements on the semiconductor substrate are optically coupled to the optical device.
When a semiconductor laser is used as an optical functional element, a semiconductor laser of the end face emission type is generally adopted. Therefore, if such an end face emission type optical functional element is mounted on the mirror-finished surface of a semiconductor substrate, light from this optical functional element is emitted in a direction parallel with the mirror-finished surface of the semiconductor substrate, and the optical device, optically coupled to the semiconductor substrate to receive the emitted ray, is supported at the end face of the semiconductor substrate, which has the optical functional element mounted thereon, to form a module of a compact structure.
Because the above-mentioned end face of the semiconductor substrate is a cleavage plane and is not mirror-finished like the surface of the substrate, the flatness of the end face is very bad.
Therefore, when connecting the semiconductor substrate, on which an optical element is mounted, to the above-mentioned optical device supported to the end face of the substrate, the alignment work to align the optical axis of the optical device with the optical functional elements invariably requires three-dimensional adjustment. Therefore, this alignment work is not easy.